Modulation

INDEX:-

1 MODULATION

2 DAIGRAM OF MODULATION

3 AMPLITUDE MODULATION

4 DAIGRAM OF AMPLITUDE MODULATION

5 INTER MODULATION

6 DAIGRAM OF INTER MODULATION


Modulation

A message signal usually spreads over a range of frequencies,called the signal band width.That is why message signals are also called base band signals-representing the band of frequencies of the original signal.
Suppose we wish to transmit an electric signal in the audio frequencies(AF)range(20 Hz to 20kHz) over a long distance.we can not do it,as such because of the following reasons:-

1. Size of antenna or aerial:-an antenna or aerial is needed both for transmission and reception.each anteena should have a size comparable to the wavelength of the signal,(atleast λ/4 in size),so that time variation of the signal is properly sensed by the antenna.

For an audio frequency signal of frequency ν=15k Hz,the wave length, λ=c/ ν =(3*10^8/15*10^3)=20000m.the length of the antenna= λ/4=20000/4=5000m.to set up an antenna of vertical height of 5000 m is practically impossible to construct and operate.
If transmission frequency were raised to 1M Hz ,then
λ=c/ ν=(3*10^8)/(10^6)=300m
the length of antenna would be (300/4)=75m which is reasonable.therefore there is an urgent need of converting the information into higf frequencies before transmission.
2. Effective power radiation by antenna:-theortical studies we reveal that power P radiated from a linear antenna of length l is
Pα1/l^2

As high powers are needed for good transmission,l should be small i.e. antenna length would be small ,for which wavelength λ should be small or frequency should be high.
Thus,this factor also points ou t to the need of using high frequency before transmission.
3. Mixing up of signals from different transmitters:-
Suppose many people are talking at the same time.we just cannot make out who is talking what.similarly,when many transmitters are transmitting baseband information signal simultaneously ,they get mixed up and there is no way to distinguish between them.the possible solution is ,communication at high frequency and allotting a band of frequencies to each message signal for it’s transmission s
All the reasons explained above o that theren is no mixing.this is what is being done for different radios and T.V.broadcast stations.
All the things explained above suggest that there is need for transmission at high frequencies.this is achieved by a process,called modulation ,wherein we superimpose the low audio frequency baseband message or information signals(called the modulating signal)on a high frequency wave(called,the carrier wave).the resultant wave is called the modulated wave,which nis transmitted.
In the process of modulation,some specific characteristics of the carrier wave is varied in accordance with the information or message signal .the carrier wave may be
(1)continuous(sinusoidal)wave,or
(2)pulse which is discontinuous.

Modulation Diagram:-



F1

F2



F1 + F2



Summation of F1 and F2 (Linear)
In a Linear System, when one sinusoid is Superimposed upon another, neither sinusoid is affected, and no frequencies are generated.



F1 x F2




Modulation/Demodulation is a Nonlinear Process where Two Sinusoids are Multiplied (F1 x F2).
The Product of this Multiplication--in the Time Domain--is a wave whose Amplitude is:
a(X) = a(F1) X a(F2).
However, in the Frequency Domain there is an Addition and Subtraction of Frequencies, i.e.,
F1 + F2, F1 - F2, F2 + F1, F2 - F1, etc.
In practical terms, the Amplitude of one of the two sinusoids is held to a Constant Value; therefore, the useful results of this process are only the Derived Frequencies.
________________________________________
Modulation per se is used to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A bandlimited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The bandlimited message is preserved, i.e., every frequency in that message is scaled by a constant value
















Amplitude-Modulation



In the design of an AM transmitter there are two ways to go:
(1) The low level generation of AM (DSB + Carrier) and the progressive amplification of that RF signal with the final stage being a Linear RF amplifier--Class AB.
In the case of the low level modulation approach, one could use either a 2 quadrant or 4 quadrant multiplier as the modulator.
TOP

The difference being: with the 2 quadrant multiplier, negative modulation of greater than 100% causes severe distortion as well as interference on adjacent bands. This is due to the carrier being cut off when the 2 quadrant multiplier cannot furnish any output for negative values of the modulating signals, hence the RF output becomes a pulsed spectrum.

If, however, a 4 quadrant multiplier is used, negative modulation starts to appear as a double sideband suppressed carrier--or in this case, reduced carrier.
(2) The second method is the progressive amplification of only the Carrier Wave with the output stage being, a more efficient, Class C (non-linear) RF amplifier; the modulation is introduced as a very high level audio signal at the final stage --more precisely, the positive plate supply of the RF "Final" Amplifier is made to vary as the modulation audio input signal.
The High Level modulation cannot handle negative modulation of greater than 100%. As with the 2 quadrant multiplier in the first approach, the carrier is cut off during negative peaks that exceed 100% negative modulation.
Most commercial AM and FM transmitter output stages--called "Finals"--use Class "C" amplifiers.
Other transmitters, like Television (visual), SSB, etc., use "Linear Amplifiers," Class AB1 or AB2, which are a combination of Class A and Class B (both being much less efficient than the Class C amplifier).



INTERMODULATION



Intermodulation is a Special Case where two (or more) sinusoids effect one another to produce undesired products, i.e., Unwanted Frequencies. Again, this can only occur when both waves share the same NonLinear device.


To Clarify: What is a Nonlinear Device? It is Any Active Device
[1]. In normal designs, radio receivers, Stereos, etc., Intermodulation is not a problem. However, when these systems are subjected to Excessive Signal Level Input the active devices in the "Front End" are driven out of their Linear Operating Regions--into or near--Saturation and/or Cutoff, where they become, in effect, "Modulators."
[1] Active Devices: Transistors, Diodes, ICs, etc.
Passive devices: Resistors, Capacitors, Inductors, etc.





Intermodulation
F1

F2



F1 + F2



Summation of F1 and F2 (Linear)



F2 x F1

F2 + F1

F2 - F1



Products Resulting from F1 and F2 (Nonlinear)